H3C Technologies H3C S10500 Series Switches - Manual (Page 72)

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address field of the frame. If they are the same, the DHCP relay agent decides this request as valid
and forwards it to the DHCP server; if not, it discards the DHCP request.
Follow these steps to enable MAC address check:
To do… Use the command… Remarks
Enter system view system-view —
Enter interface view
interface interface-type
interface-number
—
Enable MAC address check dhcp relay check mac-address
Required
Disabled by default.
NOTE:
DHCP relay agents change the source MAC addresses when forwarding DHCP packets. Therefore, you
can enable MAC address check only on a DHCP relay agent directly connected to DHCP clients.
Otherwise, valid DHCP packets may be discarded and clients cannot obtain IP addresses.
Enabling offline detection
The DHCP relay agent checks whether a user is online by learning the ARP entry. When an ARP entry is
aged out, the corresponding client is considered to be offline.
With this function enabled on an interface, the DHCP relay agent removes a client’s IP-to-MAC entry
when it is aged out, and sends a DHCP-RELEASE message to the DHCP server to release the IP address
of the client.
Follow these steps to enable offline detection:
To do… Use the command… Remarks
Enter system view system-view —
Enter interface view
interface interface-type
interface-number
—
Enable offline detection dhcp relay client-detect enable
Required
Disabled by default.
NOTE:
Removing an ARP entry manually does not remove the corresponding client’s IP-to-MAC binding. When
the client goes offline, use the undo dhcp relay security command to remove the IP-to-MAC binding
manually.
Configuring the DHCP relay agent to release an IP
address
You can configure the relay agent to release a client’s IP address. The relay agent sends a
DHCP-RELEASE message that contains the specified IP address. Upon receiving the DHCP-RELEASE
message, the DHCP server releases the IP address; meanwhile, the client entry is removed from the DHCP
relay agent.